The minimum (or lowest) safe altitude is used in aviation to designate an altitude level deemed safe over a particular geographic region. Generally, the minimum safe altitude is an altitude level that incorporates a safety buffer above the obstacles and/or terrain within a particular geographic region proximate to a route that an aircraft may travel. The minimum safe altitude is intended to ensure clearance over obstacles and terrain during flight.
Often, the minimum safe altitude is determined based on criteria provided by a governmental or regulatory organization. For example, in the United States, the Federal Aviation Administration defines criteria for the minimum safe altitude in the Federal Aviation Regulations. The regulations may provide a specific safety buffer for determining the minimum safe altitude for a region, for example, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of four nautical miles. By flying an aircraft at or above the minimum safe altitude, the pilot complies with the terrain and obstacle clearance requirements for the particular geographic region.
For instrument-based operations in accordance with instrument flight rules (IFR), the minimum safe altitudes are often published on aeronautical charts for various airways, routes, and approaches for a given navigation reference point. These aeronautical charts are published as printed procedures or charts, also known as approach plates. Depending on the navigational reference point, these paper charts often include a Minimum Sector Altitude (MSA) or Terminal Arrival Area (TAA) which depicts the minimum safe altitude for various sectors or regions surrounding the navigational reference point.
Generally, a pilot must maintain copies of these printed charts and utilize the proper chart during flight. After identifying the proper chart, the pilot must locate the appropriate sector that corresponds to the current location of the aircraft, the corresponding minimum safe altitude for the sector, and then compare it to the current altitude of the aircraft to determine if the minimum safe altitude criterion for the current sector is met. The published charts are oriented north-up and often require a pilot to manually rotate and position the chart to orient it based on the current heading of the aircraft. The minimum safe altitude is often located in the corner of the chart, or in the case of a TAA, spread out at various locations on the chart, making it difficult to locate the minimum safe altitude for a given sector. Additionally, these charts often include a variety of other information relating to the given navigational facility, further obfuscating the desired minimum safe altitude information. Thus, using and maintaining the printed charts is both cumbersome and confusing.